In a semiconductor exposure apparatus, the accuracy of alignment of a reticle and a wafer is significant in the performance, which directly influences the apparatus capability. In the exposure apparatus, a circuit pattern drawn on a reticle must be precisely overlaid on each shot area pattern on a wafer, and thus, different circuit patterns must be overlaid in multiple layers on the wafer. To obtain such high overlay accuracy, it is necessary to always accurately align the reticle and the wafer. Generally, in the exposure apparatus, the respective wafer and reticle substrates are aligned with their respective stages for high-accuracy alignment therebetween. For this purpose, the accuracy of alignment of the reticle and wafer stages is also significant.
For example, alignment of a reticle with the reticle stage is made by overlaying a reticle mark provided on a reticle lower surface on a reference mark provided on the reticle stage and measuring the overlaid marks. That is, the reticle is placed on the reticle stage, then the reticle and/or the reticle stage is moved to a position where a predetermined relative positional relation can be obtained between the reticle marks and the reference mark. Thereby, alignment is made.
Conventionally, a system to realize the above alignment has a construction as shown in FIG. 1. A reticle 5, supplied from a reticle transfer robot 1, is aligned based on an outer form shape by using thrust pins 3 in a reticle holder 2. Then, the reticle 5 is supplied by a reticle exchange hand 4 onto a reticle stage 10, and is aligned with the reticle stage 10. Numerals 20 and 21 denote measurement cameras and illumination LEDs for image sensing the reticle mark and a reference mark. An image processing unit 31 measures the positions of these marks using the obtained image data. The result of position measurement is used for reticle alignment. In this example, the result of the measurement is reflected in a reticle stage driver 11 through a control unit 32. A reticle holder 12 on the reticle stage 10, in the aligned state, holds the reticle 6. Thereby, the alignment of the reticle is realized.
However, in an initial status of the above alignment, if the positions of the reference mark and the reticle mark are unknown, a search for the mark positions must be performed. Thus, the throughput of the apparatus is seriously delayed. Generally, the position of the reference mark of the stage can be grasped by the apparatus itself. However, the position of the reticle mark differs by each reticle. Accordingly, to improve the throughput of the apparatus, rough alignment, called prealignment, is performed so as to find the reticle mark position.
Japanese Published Unexamined Patent Application No. 2000-349022 proposes a system to efficiently realize this prealignment. According to the system, the prealignment is performed in parallel with setup processing such as inspection of reticle contamination and generation of inventory. Thereby, the throughput is improved. However, in this system, as a four-cell detector fixed to a rotary part of a reticle exchange hand is used as a means for measurement of prealignment, the accuracy of alignment is the same on the reticle supply side and the reticle stage side. Further, in this system, it is difficult to attain high accuracy.